The present invention relates to a device for separating ions according to differences in their ion mobility as a function of electric field strength. The present invention also relates to a mass spectrometer, a method of separating ions according to differences in their ion mobility as a function of electric field strength and a method of mass spectrometry. The preferred embodiment relates to an asymmetric electric field ion mobility separator or spectrometer comprising a stacked array of electrodes. Ions are retained within the device by the application of an AC or RF voltage to the stacked array of electrodes.
The mobility K of an ion in a gas under the influence of an electric field E can be considered as being independent of the applied electric field under conditions wherein the energy gained by the ion from the electric field is negligible compared with thermal energies. Such conditions are met when the ratio of the strength of the applied electric field E to the neutral gas number density N of the gas is relatively low. However, if the strength of the electric field is increased or if the neutral gas number density is decreased then the mobility of an ion may then be observed as being dependent upon the ratio of the electric field strength to the neutral gas number density E/N. The mobility of the ion is observed as having a dependence as follows:
                              K          ⁡                      (                          E              N                        )                          =                              K            0                    ⁡                      [                          1              +                              α                ⁡                                  (                                      E                    N                                    )                                                      ]                                              (        1        )            wherein K0 is the mobility of the ion when the ratio E/N is relatively low and α(E/N) is a function representing the dependence of the mobility of the ion as the strength of the applied electric field increases.
A knowledge of the dependence of the mobility of an ion with electric field strength prompted development of the first differential ion mobility analyser by Buryakov et al. as disclosed in International Journal of Mass Spectrometry and Ion Processes 128 (1993) pp 143-148. The differential ion mobility analyser developed by Buryakov operated by separating ions according to differences in the mobilities of ions under low and high strength electric fields.
The device developed by Buryakov et al. is shown schematically in FIGS. 1A and 1B. The device comprised a pair of parallel electrodes 2a,2b. A flow of gas 5 was arranged to pass between the two electrodes 2a,2b and ions which were to be separated were arranged to be entrained in the flow of gas 5. An asymmetric potential difference or voltage waveform 3 was arranged to be maintained between the electrodes 2a,2b. The asymmetric potential difference or voltage waveform 3 which was applied to the electrodes is shown in FIG. 2 and comprised a relatively high positive voltage Vhigh for a relatively short period of time Thigh followed by a relatively low negative voltage Vlow for a relatively long period of time Tlow. The asymmetric potential difference or voltage waveform 3 was arranged such that the product Vhigh×Thigh equalled the product Vlow×Tlow. Consequently, if the mobility of an ion when the electric field was relatively low was the same when the electric field was relatively high then the average trajectory of the ion through the device could be expected to remain substantially parallel to the electrodes 2a,2b. The ion would therefore be expected to be onwardly transmitted through the device as shown in FIG. 1A.
If the mobility of the ion varied with electric field strength then the ion would then be expected to drift towards one or other of the electrodes 2a; 2b. The ion would therefore ultimately become lost to the system by hitting one of the electrodes 2a; 2b. This is shown in FIG. 1B. However, by applying a DC compensation voltage 4 to one of the electrodes 2a; 2b the drift of the ion towards one of the electrodes 2a; 2b can be compensated for. By appropriate setting of the DC compensation voltage 4 it is possible to arrange for ions having a specific ion mobility to be onwardly transmitted by the device whereas other ions will drift towards one of the electrodes 2a,2b and will become lost to the system.
Known differential ion mobility analysers do not confine ions within the analyser and therefore operate at atmospheric pressure since at atmospheric pressures the rate of ion diffusion is lower than at sub-atmospheric pressures. Accordingly, the loss of ions as they pass through the ion mobility analyser is minimized. If the gas pressure were to be reduced to sub-atmospheric pressures then ion diffusion would then become an important loss mechanism and the ion mobility analyser would suffer from unacceptable losses of ions.
A disadvantage of known ion mobility analysers is that since they need to operate at atmospheric pressures then high voltage RF generators are also required in order to provide an asymmetric waveform which has a high enough peak amplitude in order to be able to generate an asymmetric voltage waveform which can enable high-field mobility effects to be observed.